+'GetExtendedMetadata': Get extended file metadata for the API
+&$combinedMeta: Array of the form: 'MetadataPropName' => array(
+'value' => prop value, 'source' => 'name of hook' ).
+$file: File object of file in question
+$context: RequestContext (including language to use)
+$single: Only extract the current language; if false, the prop value should
+be in the metadata multi-language array format:
+mediawiki.org/wiki/Manual:File_metadata_handling#Multi-language_array_format
+&$maxCacheTime: how long the results can be cached
+